1. Field of the Invention
This invention relates to flicker reduction for application with signals played back by a digital video tape recorder and, more particularly, to a technique for reducing flicker which otherwise would be disturbingly present in a television picture displayed on a large screen display in response to a conventional video signal, such as a PAL or SECAM television signal that may be reproduced.
2. Description of the Prior Art
Color television signals typically are transmitted with field intervals having the repetition rate of 60 Hz or 50 Hz. Typical of the former is the NTSC television signal and typical of the latter are the PAL and SECAM television signals. Although not easily discernible in television pictures displayed from those television signals having the 60 Hz field repetition rate, it has been found that when a video picture is displayed on a large screen display in response to a PAL or SECAM signal, flicker becomes conspicuous. Such flicker is annoying and degrades the overall quality of the displayed large-screen picture. Flicker reduction apparatus has been proposed in which the field repetition rate of the color video signal is doubled, thereby displaying one hundred frames per second rather than the conventional fifty frames per second.
Of course, the doubled field repetition rate must be synchronized with the conventional field repetition rate. In the aforementioned proposal, the beginning of a field interval is detected, as by sensing the usual vertical synchronizing signal, and then an intermediate vertical synchronizing signal is generated after a predetermined number of line intervals in the field have been sensed or, alternatively, after a predetermined amount of time (equal to one-half the field interval) has passed, as shown in FIGS. 4A and 4B. While this proposal is satisfactory for television signals having fixed, uniform field intervals, such as broadcast television signals or signals reproduced during the "normal" playback operation of a video recorder, difficulties are encountered when the reproduced field intervals are not uniform.
For example, if a video recorder is operated in a special playback mode, such as slow motion or still motion playback, the reproduced vertical synchronizing signal is not uniform. As a numerical example, the field interval of a PAL or SECAM television signal is equal to 312.5 horizontal line intervals, or 312.5H. In the aforementioned special playback mode, the reproduced field intervals may vary in duration such that, in sequence, those field intervals are 313.5H, 315.5H, 313.5H, 315.5H, and so on, as shown in FIG. 5A. In the flicker reduction proposal mentioned above, the intermediate vertical synchronizing signal is generated at a time corresponding to one-half the conventional field interval. Thus, the intermediate vertical synchronizing signal is generated at 312.5(H)/2. As a result, the field interval of 313.5H is divided into two subfield intervals of 312.5(H)/2 and 314.5(H)/2. Likewise, if the reproduced field interval during the special playback mode exhibits a duration of 315.5H, the intermediate vertical synchronizing signal is generated after one-half the duration of the conventional field interval, or after 312.5(H)/2, resulting in the next subfield interval having a duration of 318.5(H)/2. Thus, although the field repetition rate of the flicker-reduced video signal is twice the field repetition rate of the original, reproduced video signal, the so-called double field intervals are not uniform, as shown in FIG. 5B. This non-uniformity in the double field vertical synchronizing signals for the flicker-reduced television signals causes vertical jitter.
A similar disadvantage occurs when a video recorder is operated in a forward or reverse picture scan mode. For example, and with reference once again to the PAL or SECAM television signal, operation of a video recorder in its forward picture scan mode may result in video signals having field intervals on the order of 296H, as shown in FIG. 6A. In accordance with the flicker reduction proposal mentioned above, an intermediate vertical synchronizing signal is produced at a location corresponding to one-half the conventional field interval, or after 312.5(H)/2. As a result, the next-following subfield interval is substantially reduced and is equal to 279.5(H)/2. Thus, the flicker-reduced video signal having the double field repetition rate is provided with sequential field intervals of durations 312.5(H)/2, 279.5(H)/2, 312.5(H)/2, 279.5(H)/2, and so on, as shown in FIG. 6B. When the video recorder is operated in its reverse picture scan mode, the durations of the field intervals of the reproduced PAL or SECAM video signals are on the order of about 330H, as depicted in FIG. 7A. Using the aforementioned flicker reduction proposal, the intermediate vertical synchronizing signal is produced after a duration of 312.5(H)/2, and the next subfield admits of a duration equal to 347.5(H)/2, as shown in FIG. 7B. Hence, the flicker-reduced video signal exhibits the double field repetition rate with successive subfield intervals equal to 312.5(H)/2, 347.5(H)/2, 312.5(H)/2, 347.5(H)/2, and so on. (It will be appreciated that the term "subfield" refers to an interval which, in the flicker-reduced video signal having twice the field repetition rate, contains a full field of video information.)
In the forward and reverse picture scan modes, the drastically changing duration of successive subfield intervals results in considerable vertical jitter.
Another difficulty is found when using the aforementioned jitter reduction technique. As is usual in video signal playback devices, when the scanning of the record medium is changed over from one transducer, or head, to another, a head-switching signal is superimposed onto the video signal. This head-switching signal is illustrated in FIG. 8A as signal S and normally is disposed in that portion of a field interval in which useful video information is not present. This non-video portion is referred to as the overscan area. Hence, the presence of the superimposed head-switching signal S is not viewed. In the aforementioned flicker reduction proposal, the video information included in a conventional, or reproduced field is repeated in two successive subfields of the flicker-reduced video signal, such as shown in FIG. 8B. Now, however, the head-switching signal which is superimposed onto the flicker-reduced video signal is located at substantially the same position (relative to the real time axis) as was the head-switching signal in the reproduced video signal. Since the flicker-reduced video signal exhibits a field repetition rate that is twice that of the originally reproduced video signal and, thus, exhibits a field interval that is one-half that of the original, the location of the head-switching signal S in the flicker-reduced video signal precedes the vertical synchronizing signal by twice the number of line intervals included in the flicker-reduced video signal. For example, if a head-switching signal S preceded the vertical synchronizing signal by six line intervals (6H) in the original video signal, as shown in FIG. 8A, the head-switching signal superimposed onto the flicker-reduced video signal now precedes the vertical synchronizing signal by twelve line intervals 12H'. (It is recognized that the duration of a line interval H' in the flicker-reduced video signal is one-half the duration of the line interval H in the original video signal.) This head-switching signal S now occurs in a line interval containing useful video information. That is, the head-switching signal no longer is concealed in the overscan area of the video signal but, rather, produces noise and distortion in the displayed television picture.